The present invention relates to castors, such as may be used on domestic and office furniture, industrial equipment, trolleys and the like.
The majority of castors in present use are of the swivel-wheel type, essentially comprises a wheel mounted on a horizontal or substantially horizontal axle which in turn is supported for turning about a substantially vertical swivel axis. Such castors, though highly versatile and efficient if well designed, do have certain inherent drawbacks. One drawback is that the bearings, especially the vertical swivel bearings are subject to non-axial forces which tend to accentuate wear and require heavy duty ball or roller bearings if wear is to be minimised and bearing life to be prolonged. Another problem is the need to provide and maintain adequate lubrication for the bearings. A further problem is that since the swivel (vertical) axis is normally offset in a vertical plane with respect to the horizontal axis, any load supported on the castor will generate a turning moment about the vertical axis and thus an extremely rigid connection must be made between the castor and the article to which it is mounted in order to withstand the resulting side forces.
To offset the aforementioned drawbacks, modern swivel castors are ruggedly built with heavy duty bearings and high quality materials. This however makes such castors relatively expensive to manufacture and replace.
Another drawback inherent in a swivel castor is that it will tend to align itself in the particular direction in which it is moving, so that when it is intended to suddenly shift the article through an angle of, say, 90.degree., the castor will tend to resist this change in direction. Thus, in general, articles mounted on swivel castors are awkward to move around corners.
To overcome the above mentioned drawbacks, a number of so-called "ball-castor" designs have previously been proposed. In this text a ball castor is one in which the main rolling element is a ball as compared with the wheel of a swivel castor. A ball castor is inherently symmetrical and does not require a vertical swivel axis. Furthermore, the load (i.e. weight of article supported on the castor) acts vertically through the axis of the ball and therefore the castor is not subject to large side forces, unlike the case with a swivel castor in which the load does not act vertically through the axis of the wheel. Thus, in principle, a ball castor is free from a number of drawbacks inherent in a swivel castor. On the other hand, there are certain difficulties associated uniquely with the design of a ball castor in practice. A main difficulty is that of supporting the ball in a manner to reduce friction as much as possible, but at the same time to maintain the ball in a stable relationship with its support. This problem arises from the fact that the ball must be capable of rotating freely in any direction about a horizontal axis and thus the means for supporting the ball must be capable of doing so with the least possible friction or resistance to the rotation of the ball. Unlike the wheel of a swivel castor, a ball has no axle or bearings and must be supported solely by some member or members in contact with its surface.
U.S. Pat. No. 78,850 to Wilkinson, patented June 9, 1868 illustrates the simplest possible arrangement for supporting a ball within a socket. This arrangement comprises stationary bearing surfaces which contact the surface of the ball and transmit the load thereto. This arrangement results in excessive friction between the ball and bearing surfaces, since these can only slide with respect to each other. The resultant friction makes it difficult to move the castor on a surface under load and can also cause the ball to jam and stick in its socket. Such a design is therefore incapable of giving a performance comparable to that of a conventional swivel castor.
U.S. Pat. No. 601,726 to Boveroux and patented Apr. 5, 1898 illustrates a castor design in which an attempt has been made to overcome the problem of friction with stationary bearing surfaces, by replacing these with ball bearings in a race. Unfortunately, such an arrangement can only materially reduce friction about the vertical axis of the main ball. The supporting ball bearings cannot significantly reduce rolling friction about a horizontal axis of rotation and therefore this design does not overcome the problem inherent with stationary bearing surfaces.
U.S. Pat. No. 992,290 to Taylor patented May 16, 1911 teaches rollers to support the main ball of a castor and this arrangement significantly improves over the previous designs referred to above. The rollers are supported on horizontal bearings and are arranged about the vertical axis of the ball and support the ball at points of contact near to the top of the ball, within a socket. Such an arrangement cannot eliminate friction, that is, resistance to rolling of the ball, but it can significantly reduce this, in principle, compared with stationary bearing or ball bearing support arrangements. In a roller supported castor as taught by Taylor, it will be seen that for any orientation of the ball rolling axis, the axes of some of the rollers will be aligned (parallel with) or approximately aligned with the ball rolling axis, while the axes of the remainder of the rollers will be substantially out of alignment with, or offset with respect to, the rolling axis of the ball. Those rollers having their axes aligned or substantially aligned with the axis of the ball will tend to roll freely with the ball and thus impose minimum resistance. On the other hand, those rollers with their axes offset with respect to the rolling axis of the ball will tend to slide and contribute maximum resistance to the movement of the ball. The net resistance, therefore, to the rolling of the ball will be determined mainly by the sliding resistance due to those rollers with their axes offset with respect to the rolling axis of the ball. This applies generally to any arrangement utilising rollers to support a castor ball as disclosed also in, for example, U.S. Pat. No. 9464 to Hinton, patented Dec. 14, 1852 and British Pat. No. 264298 to Craymer.
Although in principle the use of rollers for supporting the ball in a ball castor greatly reduces rolling resistance to the ball compared with other methods of support, in practice it has been found exceedingly difficult to reduce rolling resistance sufficiently to the point where the castor can be used on practically any smooth surface without any sticking of the ball in its socket and sliding on the smooth surface. Thus, despite the use of rollers for supporting the ball, the ball has been found to jam or stick in its socket on some smooth surfaces or, under certain loads, there has been a tendency for the ball to be forced to one side of the castor against the inside wall of the castor. At the same time rolling resistance has been relatively high compared with that of a conventional swivel castor and consequently, despite the inherent advantages mentioned previously of the ball castor over a swivel castor, the defects of the known ball castors have been such that their performance has compared unfavourably with that of an ordinary swivel castor.